Thirty to forty percent of the human population develop an ocular refractive error requiring correction by glasses, contact lens, or surgical means. Refractive errors result when the optical elements of the eye, the cornea and the lens, fail to image light directly on the retina. If the image is focused in front of the retina, myopia (nearsightedness) exists. An eye which focuses images behind the retina is said to be hyperopic (farsighted). An eye which has power that varies significantly in different meridians is said to be astigmatic. The focusing power of the eye is measured in units called diopters. The cornea is responsible for about two-thirds of the eye's 60 diopter refracting power. The crystalline lens contributes the remainder.
The search to find a permanent cure of refractive errors, including myopia, hyperopia, or astigmatism has gone on for many years. An effective, safe, predictable, and stable correction for these ametropias would have enormous social and economic impact.
Ophthalmologists have derived a number of surgical procedures which attempt to correct refractive errors. None of these techniques have gained widespread acceptance because the procedures generally have unpredictable outcomes and undesirable side effects such as glare, fluctuating vision, and corneal scarring. Unpredictable refractive outcome is the primary reason radial keratotomy has not been accepted by a majority of ophthalmologists as an effective treatment for myopia. Two other recent techniques to correct ametropias are briefly described below.
Epikeratophakia (meaning lens on top of the cornea) is a technique in which a donor cornea, termed a button, is used to make a lens which is sewn or glued to the surface of the patient's cornea in an attempt to correct his refractive error. This procedure involves freezing the donor corneal tissue and grinding the posterior surface to the desired optical configuration. This donor lenticule is then usually sewn into a groove in the patient's cornea cut by a circular mechanical trephine. Epikeratophakia is used primarily in patients who have had cataract operations and in whom an intraocular lens is contraindicated, such as children or older patients with certain eye diseases, and has also been used to correct for myopia.
There are several problems with epikeratophakia which render its routine use for patients with refractive errors unsatisfactory. The procedure is highly unpredictable and errors in refraction exceeding thirty percent of the expected change are common. Decrease in the patient's best correctable visual acuity is also encountered following epikeratophakia. The refractive change achieved by epikeratophakia is not adjustable, and patients who are grossly over or under corrected must have the button removed and replaced.
A second technique to correct refractive errors, termed photorefractive keratectomy, has recently been developed. Energy generated by a pulsed argon fluoride excimer laser at a wavelength of 193 nm causes a precise removal of corneal tissue without adjacent thermal or mechanical damage. Several optical laser delivery systems have been described which attempt to achieve a controlled etching of the anterior cornea to the desired refractive curvature. Unfortunately, three major shortcomings render this technique clinically unacceptable for some patients: scarring, unpredictable outcome, and refractive instability.
Since this procedure is performed directly in the visual axis, any scarring is unacceptable. Several investigators have demonstrated relatively dense opacification of the cornea in animals following a reprofiling procedure with the 193 nm excimer laser. Although the scarring may be reduced by improvements in the optics of the delivery systems, there will remain very significant risks associated with irreversible treatment of the central optical axis of the eye.
A second problem with photorefractive keratectomy is the unpredictable refractive outcome of the technique. The final curvature of the ablated cornea will be influenced by stress and strain distribution changes occurring following ablation. Since the eye is a pressurized sphere, removal of some of its anterior surface causes the remaining tissue to accept a higher load of wall tension caused by the intraocular pressure. The net affect of this redistribution will cause the ablated cornea to shift in an anterior direction following photorefractive keratectomy, inducing a steepening of the optical zone. The extent of this shift will vary with the intraocular pressure, the patient's age, and tissue characteristics and therefore can cause significant variations in optical outcome.
Another serious problem with directly reprofiling the central area of the anterior corneal surface is the inability of the procedure to achieve a stable outcome. The majority of the thickness of the cornea is termed the stroma and consists of water, collagen fibers, a matrix substance, and numerous cells called keratocytes. The keratocytes, which reside between Bowman's layer and Descemet's membrane, help produce and maintain the collagen structure of the corneal stroma. These cells are also responsible for wound healing following corneal injury. When Bowman's layer is violated, these cells produce new collagen and attempt to reform the injured or disrupted fibers. The cornea heals in a very slow fashion with the keratocytes laying down and remodeling new collagen over many months to years.
During photorefractive keratectomy, a certain depth of the anterior cornea is ablated, depending on the refractive change desired. The more refractive change needed, the deeper the required ablation. Despite the fact that the collagenous surface re-epithelializes normally after laser ablation, the keratocytes begin to produce new collagen in an attempt to restore the thinned stroma. This regeneration of new collagen (which is highly variable) in the anterior stroma will cause the corneal curvature to continually change as new collagen is added.
In addition to stroma collagen regeneration, investigators have noted a compensatory thickening of the epithelium following photoablative keratectomy. This hyperplastic epithelium, which may result from wound healing mediators, will vary in its thickness over time and contribute to the instability or the refractive outcome. Since the processes of stromal collagen regeneration and epithelial hyperplasia occur over a long period of time, the refractive power of the eye will be constantly fluctuating for many months or years following the procedure.